After only a somewhat recent introduction, optical fiber has had a meteoric rise as the predominant means of transmission media in voice and data communications. Optical fiber is manufactured by drawing the fiber from a preform which is made by any of several well known processes. Afterwards, or as part of a tandem process, the drawn fiber is coated, cured, measured and taken up, desirably in an automatic takeup apparatus, on a spool to provide a package. Typically, an optical fiber has a diameter on the order of 125 microns, for example, and is covered with a coating material which increases the outer diameter of the coated fiber to about 250 microns, for example.
A package of optical fiber is used in operations such as ribboning, cabling, and rewinding and is used to ship optical fiber to other companies which further process the fiber. The optical fiber typically is used in voice and data communications systems, both commercial and military. Optical fiber may be used in weapons systems in which it is used for guidance and for data communications. Such uses include communications between aircraft, between an aircraft and a ship, and between a projectile, such as a missile or torpedo, and a control station at a launch site, for example. Optical fiber provides the advantages of increased data bandwidth, reduced weight and greater range than wire-guided systems of the prior art.
One optical fiber application in a weapons system involves the packaging of a continuous length of optical fiber on a bobbin which is positioned inside a vehicle such as a torpedo, for example. Such a vehicle commonly is referred to as a tethered vehicle. One end of the fiber is attached to operational devices in the vehicle, whereas the other end of the fiber is connected to a control or communications station at a launch site. During and after launch, two-way communication with the vehicle is conducted.
There are, however, certain disadvantages, not present in other forms of communication, in using optical fiber for guiding tethered vehicles. Optical fiber is less robust than metallic conductors, rendering it subject to breakage. Aside from breakage, optical fiber communication performance may be degraded by microbends in the fiber which are generated by bending or by other stresses to which the fiber may be subjected. Such damage to an optical fiber not only reduces the long-term durability of the fiber, but also causes losses in the strength and in the content of the optical signal. Likewise, physical or optical integrity may be affected adversely by any sharp bends which are experienced as the fiber pays out from its packaged configuration.
In order to use such an arrangement for a tethered vehicle, there must be provided a reliable and compact package of the optical fiber which may be disposed within the vehicle and which will permit reliable deployment of the optical fiber during the flight of the vehicle. The use of metallic conductors for guidance or control of launched vehicles is known. Although the art teaches the use of bobbins on which a metallic conductor is wound, the fragility of optical fiber requires specialized treatment that facilitates the unwinding of the optical fiber from its bobbin at a relatively high rate of speed.
Another problem in the optical fiber guidance of tethered vehicles relates to the successful unwinding of the fiber from a bobbin as the bobbin is propelled along with the vehicle. In optical fiber packages for use in tethered vehicles, as many as at least thirty layers of optical fiber are wound on a guiding structure. The leading end of the optical fiber is connected to a guidance system for controlling the path of travel of the vehicle. It becomes important for the optical fiber to be payed off from the bobbin without the occurrence of snags, or tight bends, otherwise the fiber may break or the signal may be attenuated and the control system rendered inoperable. Contributing to the successful payout of the optical fiber is a precision wound package. Not only must the convolutions be wound with precision, they also must remain in place as wound during handling and during deployment. In other words, the optical fiber package must be a highly stable one.
During storage and transport of the bobbin, mechanical stability is most important to the integrity to the wound package, thereby maintaining the package in a ready condition for deployment. During deployment, both mechanical and optical effects are significant. The package must permit a helical pattern of payout at potentially high speeds, possibly approaching or exceeding Mach 1. Also, microbending in the layers of undeployed fiber on the bobbin during deployment can affect adversely optical performance.
The foregoing problems are exacerbated when optical fiber is contemplated for tethered submersibles such as tethered torpedos. Optical fiber in such uses is destined to experience significant tension, given the travel in a liquid instead of in air. Further, the optical fiber undergoes a high degree of environmental abuse because of its passage most likely through strong currents and through surfs. Presently, such tethered vehicles are guided by metallic conductors which extend from the torpedo to the launching body.
What is needed and what has not been available in the prior art is a sheathed optical fiber which is robust and includes suitable mechanical protection for the optical fiber thereof. Further what is sought is a bobbin of precision wound optical fiber in which the convolutions of fiber are held together in a stable package that permits payout at relatively high speeds in an underwater environment.